ENH: Improve handling of optional Cartopy map features

Use the module-level __getattr__ support in Python 3.7 to dynamically
handle access to the CartoPy map features and only warn *upon use* if
CartoPy isn't installed.

src/metpy/plots/__init__.py

@@ -7,6 +7,7 @@
 
 # Trigger matplotlib wrappers
 from . import _mpl  # noqa: F401
+from . import cartopy_utils
 from ._util import (add_metpy_logo, add_timestamp, add_unidata_logo,  # noqa: F401
                     convert_gempak_color)
 from .ctables import *  # noqa: F403
@@ -25,10 +26,20 @@
 __all__.extend(wx_symbols.__all__)  # pylint: disable=undefined-variable
 __all__.extend(['add_metpy_logo', 'add_timestamp', 'add_unidata_logo',
                 'convert_gempak_color'])
-try:
-    from .cartopy_utils import USCOUNTIES, USSTATES  # noqa: F401
-    __all__.extend(['USCOUNTIES', 'USSTATES'])
-except ImportError:
-    logger.warning('Cannot import USCOUNTIES and USSTATES without Cartopy installed.')
 
 set_module(globals())
+
+
+def __getattr__(name):
+    """Handle warning if Cartopy map features are not available."""
+    if name in cartopy_utils.__all__:
+        try:
+            return getattr(cartopy_utils, name)
+        except AttributeError:
+            logger.warning(f'Cannot use {name} without Cartopy installed.')
+
+    raise AttributeError(f'module {__name__!r} has no attribute {name!r}')
+
+
+def __dir__():
+    return sorted(__all__ + cartopy_utils.__all__)

src/metpy/plots/cartopy_utils.py

@@ -56,6 +56,8 @@ def with_scale(self, new_scale):
 except ImportError:
     pass
 
+__all__ = ['USCOUNTIES', 'USSTATES']
+
 
 def import_cartopy():
     """Import CartoPy; return a stub if unable.